Midrash su Levitico 25:7

(Fol. 62) We are taught in a Baraitha: If two were on the road [in the desert], and one of them has a pitcher of water which is sufficient for one only until he may reach an inhabited place, but if both would use it both would die before reaching a village; Ben Pattira lectured that in such a case it is better that both should drink and die than one should witness the death of his comrade, until R. Akiba came and taught: (Lev. 25, 7) That thy brother may live with thee; (i.e., thy own life is preferred to the life of thy brother).