It is narrated from the Mother of the Believers, Sayyidah Aishah (may Allah be pleased with her), that he (peace and blessings be upon him) said: "Two rak'ahs prayed after using the siwak are more beloved to me than seventy rak'ahs prayed before using the siwak."
(b) Al-Waqidi cannot be used as evidence.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الطهارة / 160