Abu Hurairah (may Allah be pleased with him) narrates that the Messenger of Allah (peace and blessings be upon him) said: "If I did not fear hardship and difficulty for my Ummah, I would have commanded them to delay the ‘Isha prayer and to use the miswak with every prayer."
Hadith Referenceمسند الإمام الشافعي / كتاب الطهارة / 55
Hadith Takhrijأخرجه البخاري: الجمعة، باب السواک یوم الجمعة (887)، ومسلم، الطهارة، باب السواک (252).