Answered by Shaykh Faraz Rabbbani
Question: Is the prayer of someone who only prays Fajr prayer valid?
Answer: Walaikum assalam wa rahmatullahi wa barakatuh,
Validity (sihha) relates to fulfilling the obligatory conditions and integrals. Acceptability (qabul) relates to sincerity (ikhlas) in the specific action.
Yes, it is hoped that the Fajr would be accepted–but sincerity would also entail striving to perform the remaining prayers, as soon as possible. Every step on takes towards Allah is multiplied beyond measure by Allah.
Allah tells us in the authentic hadith qudsi, “Whoever draws close to Me by a handspan, I draw close to them by an arm’s length. Whoever draws close to Me by an arm’s length, I draw close to them by two arm’s lengthens. And whoever comes to Me walking, I rush to them.” (Muslim, Tirmidhi, and Ibn Maja)
And Allah alone gives success.
The Rulings of the Sacred Law