Sayyiduna Abu Hurayrah (may Allah be pleased with him) reports this saying of the Prophet (peace and blessings be upon him): "Everything has a choicest part, and the choicest part of the prayer is the first takbir." The narrator says: After this he mentioned the complete hadith.
Al-Bazzar recorded it; its chain contains a narrator, al-Hasan bin al-Sakan, whom Ahmad declared da'if (weak), and Ibn Hibban (may Allah have mercy on him) mentioned him in his book "al-Thiqat".
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب الصلاة / 2598