Sayyiduna Abdullah bin al-Zubayr (may Allah be pleased with them both) narrates that the Prophet (peace and blessings be upon him) said: "Before every obligatory prayer, two rak'ahs are to be offered."
Al-Tabarani recorded it in al-Mu'jam al-Kabir and al-Mu'jam al-Awsat; its chain contains Suwayd bin Abd al-Aziz, and he is da'if (weak).