It was recorded by al-Nasāʾī and Aḥmad through the line of transmission that runs: Baqiyyah said, “Bahīr ibn Saʿd narrated to us, from Khālid ibn Maʿdān, from Kathīr ibn Murrah, from him.” This isnād is sound—every one of its transmitters is trustworthy, and Baqiyyah has explicitly stated that he heard the report directly.
Al-Nawawī, may Allah have mercy on him, commented:
“The Prophet’s words, ‘the like of it,’ admit two possible meanings:
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That Allah, Exalted is He, will build for the person something that is likewise called a ‘house,’ while its spaciousness and other qualities are, by definition, of the sort ‘no eye has seen, no ear has heard, and no human heart has ever conceived.’
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That its superiority over the other houses of Paradise is comparable to the superiority of a mosque over the houses of this world.”
The first interpretation is the clearer one, and al-Ḥāfiẓ adopted it in Fatḥ al-Bārī. Allah knows best.
It must also be noted that this reward is reserved for the one who erects a mosque purely for the sake of Allah, as the wording of the ḥadith—“for Allah”—makes plain. The general proofs from the Quran and Sunnah confirm this as well.
Whoever builds for the sake of boasting, ostentation, or currying favor with the masses—as many rulers and dignitaries do—earns nothing but sin. For this reason al-Ḥāfiẓ said:
“Benefit: Ibn al-Jawzī stated, ‘Whoever writes his name upon the mosque he builds is far removed from sincerity.’ End quote. Likewise, one who builds it for a wage does not receive this promise.”