It is narrated from Sayyiduna Mutlib bin Abi Wada'ah (may Allah be pleased with him) that the Noble Prophet (peace and blessings be upon him) said: "The prayer is in sets of two rak‘ahs. After every two rak‘ahs, there is tashahhud. (Before Allah) express your poverty and neediness, and raise your hands with humility and say « اللَّهُمَّ », « اللَّهُمَّ » O my Lord, O my Lord (i.e., make supplication). Whoever does not do these things, his prayer is incomplete."
Hadith Referenceصحيح ابن خزيمه / جماع أبواب التطوع غير ما تقدم ذكرنا لها / 1212