بسم الله الرحمن الرحيم
Wa `alaykumu s-salam wa rahmatuLlah
The following are Ibn Hajar’s fiqh works:
- Tuhfat al-Muhtaj (Sharh al-Minhaj)
- al-I’ab (Sharh al-`Ubab)
- al-Imdad (Sharh al-Irshad)
- Fath al-Jawwad (Sharh al-Irshad)
- al-Minhaj al-Qawim (Sharh al-Muqaddimah al-Hadramiyyah)
- Hashiyat al-Idah (fi l-Manasik)
- al-Fatawa al-Fiqhiyyah al-Kubra
In terms of prioritisation, the last three works may immediately be dismissed: al-Minhaj al-Qawim on account of extreme brevity and restricted focus, and Hashiyat al-Idah on account of restricted focus. The Fatawa too, is not an exhaustive treatment of fiqh. Fath al-Jawwad is much shorter than Tuhfah and Sharh al-‘Ubab, and accordingly lacks the degree of sophistication and research that is fond in those two works. It is abridged from al-Imdad, but this work has not been published and I don’t know if it survives in manuscript.
This leaves Tuhfah and Sharh al-‘Ubab.
Tuhfah holds several advantages over Sharh al-‘Ubab:
- Tuhfah was completed; Sharh al-‘Ubab remains incomplete
- Unlike Sharh al-‘Ubab, Tuhfah was read by Ibn Hajar to large groups of his senior students
- Tuhfah‘s text is the Minhaj, which is much less voluminous than al-Muzajjad’s ‘Ubab; this makes Tuhfah much better suited for teaching and transmission purposes
- In Tuhfah Ibn Hajar was more concerned with tahqiq, whereas in Sharh al-‘Ubab his main objective is said to have been exhaustive coverage.
The last point is mentioned by those of our fuqaha who versified the order of reference among Ibn Hajar’s works. One version, ascribed to Ali ibn Abd al-Rahim BaKathir, runs as follows:
وشاع ترجيح مقال ابن حجر في يمن وفي الحجاز فاشتهر
وفي اختلاف كتبه في الرجح الأخذ بالتحفة ثم الفتح
فأصله فشرحه العبابا إذ رام فيه الجمع والإيعابا
The above would perhaps answer the question WHY Tuhfah came to occupy pride of place amongst Ibn Hajar’s fiqh works. Your question, however, was not WHY, but HOW. How that came about was simply by the concurrence of later generations of fuqaha. I would venture to suggest that this concurrence of theirs was motivated by reasons such as the ones stated here.