Musannaf Ibn Abi Shayba 13:1496

Abu Bakr narrated to us, he said: Waki’ narrated to us, from Hammad ibn Salamah, from Abu al-Muhazzam, from Abu Hurayrah, he said: If the owner of the capital stipulates to the Mudarib (entrepreneur) not to descend into the bottom of a valley, and he descends into it, then he is a guarantor (liable for the loss).