Mishkat al-Masabih #3056
Marriage
The Arabic includes the chain of narration (sanad) and the collector's commentary; the English translation shows the Prophet's words only.
وَعنهُ كُنَّا نَعْزِلُ وَالْقُرْآنُ يَنْزِلُ. مُتَّفَقٌ عَلَيْهِ. وَزَادَ مُسْلِمٌ: فَبَلَغَ ذَلِكَ النَّبِيَّ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ فَلم ينهنا
He said that in the period when the Qur'an was coming down they used to withdraw the penis.*
* i.e. before emission of semen, to avoid conception.
(Bukhari and Muslim.) Muslim added that the Prophet heard of that and did not prohibit it.
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About Mishkat al-Masabih
This hadith is #3056 from Mishkat al-Masabih, in the Book of Marriage.