Making Du`a' to the dead and the absent like the Prophets, Awliya' (pious people), and the like, whether alone or along with Allah, is an act of Al-Shirk-ul-Akbar (major form of associating others in worship with Allah), even if a person observes Sawm, performs Salah, and pays Zakah because Allah (Glorified be He) says:
"And invoke not besides Allâh, any such that will neither profit you nor harm you, but if (in case) you did so, you shall certainly be one of the Zâlimûn (polytheists and wrong-doers)."
Also Allah (Glorified be He) says:
Such is Allâh, your Lord; His is the kingdom. And those, whom you invoke or call upon instead of Him, own not even a Qitmîr (the thin membrane over the date-stone).
If you invoke (or call upon) them, they hear not your call; and if (in case) they were to hear, they could not grant it (your request) to you. And on the Day of Resurrection, they will disown your worshipping them. And none can inform you (O Muhammad صلى الله عليه وسلم) like Him Who is the All-Knower (of everything).
(Part No. 1; Page No. 143) There are many Ayahs (Qur'anic verses) to this effect.May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.