It is narrated from Sayyiduna Anas bin Malik (may Allah be pleased with him) that the Companions (may Allah be pleased with them) looked for water but did not find any. The Messenger of Allah (peace and blessings be upon him) said: "Come here!" I saw the Prophet (peace and blessings be upon him) place his (peace and blessings be upon him) hand in the vessel that had water in it, and then he said: "Perform ablution saying 'Bismillah' (in the name of Allah)." The narrator says: I saw the water gushing out from between his fingers, and the people kept performing ablution until the last of them had performed ablution. Thabit (may Allah have mercy on him) asked Sayyiduna Anas (may Allah be pleased with him): "How many were you at that time?" He said: "About seventy." This hadith is the most authentic one regarding saying Bismillah.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الطهارة / 191