Allah (s.w.t.) is expressly asking His creatures to find Him through “Waseela” to attain happiness.Surah al-Ma'idah Ayah 35 says, “Believers, have fear of Allah. Find the means to reach Him and strive hard for His cause so that you may have everlasting happiness.” In order to seek Allah (s.w.t.)'s forgiveness.

      He is directing His creatures to go through the Noble Messenger (s.a.w.a.s.) as explained in Surah al-Nisa' Ayah 64, “....If they ever do injustice to themselves and come to you (Muhammad) asking for Allah's forgiveness and if the Messenger also were to ask Allah to forgive them, they would certainly find Allah All-Forgiving and All-Merciful.” Similarly, Prophet Yousuf's brothers approached their father to seek forgiveness from Allah for their sins about their brother Prophet Yousuf as mentioned in Surah Yousuf Ayah 97-98, “Father, ask forgiveness for our sins; we have certainly sinned.” He said, “I shall ask my Lord to forgive you; He is All-Forgiving and All-Merciful.” On the same line, Allah in some cases restricts the intercessor that without His permission they will not seek His forgiveness as detailed in the famous “Ayat-al-Kursi” in Surah al-Baqarah Ayah 255, “.....No one can intercede with Him for others except by His permission.” Even the prophets do not dare intercede for anyone without His permission, as stated in Surah al-Anbiya Ayah 27-28.

      “These servants do not speak before He speaks . They simply act according to His orders. He knows all that is in front of them and all that is behind them. (These servants of Allah) will not intercede with Him for anyone without His permission and they tremble in awe (before His greatness).”

      Shias, when they seek assistance from the Noble Messenger (s.a.w.a.s), Imam Ali (a.s.) and other Infallible Imams (a.s.) , they do it with the full knowledge that they are alive as they were martyrs in the path of Allah and further that they are not independent of Allah but because of their status with Allah they seek them as “Waseelah” in the hope that if they recommend our case Allah may grant our wishes and duas. In Dua-e-Tawassul, we seek help from the “Fourteen Infallible” individually and repeat, “We turn towards Allah with your help, seek your intercession and advocacy before Allah, we call upon you before our request (to Allah);O intimate of Allah, stand by us when Allah sits on judgment over us.”

      The Sunni traditionalist Tirmidhi relates through his chain of narrators from Usman ibn Hunayf that a blind man came to the Noble Messenger (s.a.w.a.s.) and said, “I have been afflicted in my eyesight, so pray to Allah for me.” He (s.a.w.a.s.) told him to pray two rak'at prayer and say, “O Allah, I ask you and turn to you through my Prophet Muhammad, the Prophet of Mercy; O Muhammad, I seek your intercession with my Lord for the return of my eyesight.”

      Even after the death or martyrdom of the Noble Messenger (s.a.w.a.s.), Tabrani in his “al-Mujam al-Saghir” a man came to Usman ibn Hunayf for help and Usman once again told him follow the same supplication advised by the Messenger.

      According to the Glorious Qur'an, if a Muslim does not fulfill his obligations and adherence to the path of the Noble Messenger (s.a.w.a.s.) and his Progeny (a.s.), he will not be considered worthy of intercession, even if the Noble Messenger (s.a.w.a.s.) were to intercede on his behalf as stated in Surah al-Munafiqun Ayah 6, “It is all the same whether you sought forgiveness for them or not; Allah will never forgive them. Allah does not guide the evil-doing people.”

      It is reported that when Abdullah ibn Abbas was on death-bed, he said, “O Allah I seek to approach toward you by means of Wilaya of Ali ibn Abi Talib (Fada'il al-Sahaba by Ahmad ibn Hambal v2 page 662 #1129 and al-Riyad al-Nadhira by Muhibuddin al-Tabari v3 page 167)

Sources: the Glorious Qur'an, A Shi'ite Encyclopedia