Sayyiduna Hudhayfah (may Allah be pleased with him) said that they prayed with the Prophet (peace and blessings be upon him). Between the two prostrations he (peace and blessings be upon him) would say: "'Rabbighfir li, rabbighfir li' (O Allah, forgive me, O Allah, forgive me)", and he would make his sitting equal in length to his prostration.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 2749