Sayyiduna Abu Hurairah (may Allah be pleased with him) narrates: The Noble Prophet (peace and blessings be upon him) said: "When a person wakes up at night, he should perform two short rak‘ahs with which he should begin his prayer."
Shaykh Muhammad Ibrahim bin Basheer
Benefit: The first two units (rak‘ahs) of the Tahajjud prayer should be performed lightly; this applies to certain occasions, and at other times the Prophet sallallahu alayhi wa sallam would perform the first two units of Tahajjud for a longer duration. [صحيح مسلم : 763، 1792]
Source: Musnad al-Humaydi: Commentary by Muhammad Ibrahim bin Bashir, Page: 1014