Sunnah.com reference : Book 7, Hadith 180English translation : Book 7, Hadith 0Arabic reference : Book 7, Hadith 926
A narration by al-Bukhari has:
"He gave the land in Sadaqah (as an endowment) that must not be sold or gifted, but its produce must be spent (as Sadaqah)."
وَفِي رِوَايَةٍ لِلْبُخَارِيِّ : { تَصَدَّقْ بِأَصْلِهِ , لَا يُبَاعُ وَلَا يُوهَبُ , وَلَكِنْ يُنْفَقُ ثَمَرُهُ }   1‏ .‏‏1 ‏- البخاري برقم (2764)‏.‏