Mishkat al-Masabih 896
Rabi'a b. Ka‘b said:
I was with God’s Messenger at night, and when I brought him his water for ablution and what he required, he told me to make a request. I said, “I ask to accompany you in paradise.” He asked if I had any other request to make, and when I replied that that was all, he said, “Then help me to accomplish this for you by devoting yourself often to prostration.” 

Muslim transmitted it.
وَعَن ربيعَة بن كَعْب قَالَ: كُنْتُ أَبِيتُ مَعَ رَسُولِ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ فَأَتَيْتُهُ بِوَضُوئِهِ وَحَاجَتِهِ فَقَالَ لِي: «سَلْ» فَقُلْتُ: أَسْأَلُكَ مُرَافَقَتَكَ فِي الْجَنَّةِ. قَالَ: «أَو غير ذَلِكَ؟» . قُلْتُ هُوَ ذَاكَ. قَالَ: «فَأَعِنِّي عَلَى نَفسك بِكَثْرَة السُّجُود» . رَوَاهُ مُسلم