Qatadah narrates, through Sa'id bin al-Musayyib, from Sayyiduna Sa'd bin Ubadah (may Allah be pleased with him) that he came to the Prophet (peace and blessings be upon him) and said: "O Messenger of Allah, my mother has died; shall I give charity on her behalf?" He (peace and blessings be upon him) said: "Yes." He said: "Which charity is best?" He (peace and blessings be upon him) said: "Giving water to drink."
Hadith Referenceالمعجم الكبير للطبراني / باب السين / 5360