It is to be noted that suhoor is a “mustahab” practice, it is neither “wajib” nor “farz”, therefore, even if someone skips suhoor, their fast will be deemed valid. However, for a qaza fast, one should make the intention to fast the night prior or at least before subah sadiq (the break of dawn) occurs. In essence, if someone does not make the intention to fast (in the case of a qaza fast) before subah sadiq occurs, their fast will be deemed invalid.
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دلائل:
الھندیۃ: (200/1)
التسحر مستحب ووقتہ آخراللیل قال الفقیہ ابو اللیث وھو السدس الاخیر۔ ھکذا فی السراج الوھاج۔
و فیھا ایضا: (کتاب الصوم، 256/1، ط: دار الفکر)
وشرط القضاء والکفارات ان یبیت و یعین، کذا فی النقایۃ
الدر المختار: (380/2، ط: دار الفکر)
"(والشرط للباقي) من الصيام قران النية للفجر
Allah the Almighty knows best.
Darul ifta Al-Ikhlas, Karachi.