The time of the mustajab falls after the asar. This is the opinion of Abdullah bin Sallam, Abu Hurairah Radhiyallahu 'anhu, Imam Ahmad and some scholars.
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There are some traditions that support this opinion,
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hadith from Jabir ibn Abdillah Radhiyallahu 'anhuma, that the Prophet sallallaahu' alaihi wa sallam said,
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يوم الجمعة اثنتا عشرة ساعة, لا يوجد فيها عبد مسلم يسأل الله شيئا إلا آتاه إياه فالتمسوها آخر ساعة بعد العصر
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on Friday there is 12 hours. (There is a time when there is a Muslim who asks God at that time, surely God will give.) Seek that time at the end of the day after the asar (Abu Dawud 1048, Nasai 1389 and al-Albani).
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hadith from Abdullah bin Sallam Radhiyallahu 'anhu, he once asked the Prophet sallallaahu' alaihi wa sallam
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"We met in the Book of the Holy Book, that on Friday there is a time, if there is a servant of faith performing the prayer to coincide with that time, then pleading with Allah, then Allah will fulfill his request."
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then the Prophet sallallaahu 'alaihi wa sallam cues to me,' It's only a moment? '
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'You are right, only for a moment' replied Abdullah bin Sallam.
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Then Abdullah asked, 'When was that time'
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He replied,
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هي آخر ساعات النهار
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"That is the time at the end of the day." '
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'Is not that the time for prayer restrictions?'
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Answer the Prophet sallallaahu 'alaihi wa sallam,
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بلى, إن العبد المؤمن إذا صلى ثم جلس لا يحبسه إلا الصلاة, فهو في الصلاة
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"True, but when a servant performs a prayer (at the beginning of the asar), then he sits for the next prayer, he is calculating praying." (Narrated by Ibn Majah 1139).