Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) said: I heard the Messenger of Allah (peace and blessings be upon him) say: "When one of you performs ablution and mentions the name of Allah, his whole body becomes purified; and if someone does not mention the name of Allah while performing ablution, only the parts over which the water passed are purified. And when one of you finishes his purification, let him bear witness that there is no god worthy of worship except Allah and that Muhammad (peace and blessings be upon him) is His servant and Messenger, and then let him send blessings upon me. When he does so, the gates of mercy will be opened for him."
(b) The Shaykh says: This narration is weak; to my knowledge only Yahya bin Hashim narrated it from al-A'mash.
(c) Yahya bin Hashim is matruk al-hadith (abandoned in hadith).
(d) It has also been narrated from Sayyiduna Ibn Umar (may Allah be pleased with them both) through another chain.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الطهارة / 198