I say, and with God's success: It is a condition that I'tikaf be in a congregation mosque or a main mosque for a man, or in the house mosque for a woman; due to His saying, Glory be to Him: {And do not approach them while you are in I'tikaf in the mosques} Al-Baqarah: 187. He described them as being in I'tikaf in the mosques even though they did not engage in sexual relations in the mosques; to prohibit sexual relations there, which indicates that the place of I'tikaf is the mosque. And from Aisha, may Allah be pleased with her, she said: «The Sunnah for the one in I'tikaf is that he does not visit the sick, nor attend a funeral, nor touch a woman, nor engage with her, nor leave for a need except for what is absolutely necessary, and there is no I'tikaf except with fasting, and there is no I'tikaf except in a main mosque» in Sunan Abu Dawood 2: 333, and Sunan Al-Bayhaqi Al-Kabir 4: 321, and Musannaf Abdul Razzaq 3: 168. This applies equally to obligatory and voluntary I'tikaf; because the text is general. Refer to: Bada'i Al-Sana'i 2: 113, and Allah knows best.