ENOfficial
Umm Salama told that she heard God's messenger say, "The Mahdi will be of my family, of the descendants of Fatima." Aba Dawud transmitted it.
Source: sunnah.com
Other translations
Arabic
وَعَنْ أُمِّ سَلَمَةَ قَالَتْ: سَمِعْتُ رَسُولَ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ يَقُولُ: «الْمَهْدِيُّ مِنْ عِتْرَتِي مِنْ أَوْلَادِ فَاطِمَة» . رَوَاهُ أَبُو دَاوُد
sunnah.com