From Hudhayfah bin al-Yamaan (radiyallaahu ta'aala 'anhu), "That the Prophet (Sallallaahu 'alayhi wasallam) used to say between the two prostrations Rabbigh-fir lee, Rabbigh-fir lee.""
It was collected by Ahmad (23375), Aboo Daawood (874), an-Nasaa'ee (1145), Ibn Maajah (897), and authenticated by Shaykh al-Albaanee (rahimahullaah) in Irwaa' al-Ghaleel (2/335).
Shaykh Ibn Baaz (rahimahullaah) said: "And as for the sitting between the two prostrations, then everyone says - meaning the Imaam and the follower - Rabbigh-fir lee, Rabbigh-fir lee, Rabbigh-fir lee, 3 times or more. And that which is obligatory is one time, and the rest are sunnah." [Fataawaa Ibn Baaz (11/86-87)].
And he said (rahimahullaah): "But he makes more du'aa of seeking forgiveness in what is between the two prostrations as comes reported from the Prophet (Sallallaahu 'alayhi wasallam)." [(11/37)]
Refer to Al-Wasiyyatu bi-ba'dh as-Sunan shabhu al-mansiyyah of Hayfah bint 'Abdillaah Ar-Rasheed, checked by our brother, Shaykh Abool-'Abbaas 'Aadil Mansoor