What is the ruling for someone who intended to pray Witr three rak'ahs but then, out of forgetfulness, prayed only one rak'ah, and what should they do?

If a worshipper intends to pray Witr with three rak'ahs and mistakenly says the tasleem after only one rak'ah, then if he remembers shortly thereafter, he should complete the remaining two rak'ahs and perform the prostration of forgetfulness (sujud al-sahw) at the end of his prayer. However, if a long time has passed, he must repeat the entire Witr prayer. The determination of what constitutes a short or long interval is left to common practice and custom.