(a) A similar hadith is reported from Thawban (may Allah be pleased with him) through another chain, except that in it he added: 'wa ilayka as-salam, tabarakta ya dhal-jalali wal-ikram' (and to You is peace; Blessed are You, O Possessor of majesty and honour).
(b) Al-Walid says: I asked al-Awza'i (may Allah have mercy on him): How was the seeking of forgiveness done? He said that one should say: 'Astaghfirullah, astaghfirullah' (I seek Allah's forgiveness, I seek Allah's forgiveness).
(c) Imam Muslim (may Allah have mercy on him) recorded this narration by way of Dawud bin Rushayd, but he did not mention 'wa ilayka as-salam' (and to You is peace), and he said: 'tabarakta ya dhal-jalali wal-ikram' (Blessed are You, O Possessor of majesty and honour). Then he mentioned the words of al-Awza'i (may Allah have mercy on him).
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 3007