Yahya related to me from Malik that he had asked Ibn Shihab about the ila of the slave. He said that it was like the ila of the free man, and it put an obligation on him. The ila of the slave was two months.

حَدَّثَنِي يَحْيَى، عَنْ مَالِكٍ، أَنَّهُ سَأَلَ ابْنَ شِهَابٍ عَنْ إِيلاَءِ الْعَبْدِ، فَقَالَ هُوَ نَحْوُ إِيلاَءِ الْحُرِّ وَهُوَ عَلَيْهِ وَاجِبٌ وَإِيلاَءُ الْعَبْدِ شَهْرَانِ ‏.‏
USC-MSA web (English) reference : Book 29, Hadith 19
Arabic reference : Book 29, Hadith 1175