Spiritual Retreat: Staying Outside the Mosque for a Long Time
Practical Laws of Islam as per the teachings of Ayatullah Sistani
Ruling 1751
If a muʿtakif leaves the mosque to attend to some urgent matter but stays outside for a long time such that the form of his iʿtikāf is lost, his iʿtikāf is invalid even if he was compelled or forced to leave, or if he left because of necessity or due to forgetfulness.
-Ayatullah Sistani, Practical Laws of Islam, Spiritual Retreat (Iʿtikāf)