this is an excerpt from the vividhbharati interview ( courtsey Sujoy - the owner of the yahoogroup )
O P says that "Kishore wanted to sing ( classical-which is bakwaas according to me) but I took Rafi"!

did he not sing a different tune/story earlier? Pavan?

KCP
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KS: Nayyar saahab, Kishore Kumar saahab se to aapne bahut saare gaane
gawaaye lekin mujhe yaad hai 1958 mein aapki ek film aayi thi Ragini,
jismein Kishore-ji ne bhi abhinay kiya tha lekin unpar jo
gaana 'picturize' kiya gaya tha woh aapne gawaaya tha Mohd Rafi
saahab se.

OP: yeh isliye kiya ke woh 'classical mood' ka gaana tha, "man moraa
baawra", to woh baDaa chhaTpaTaaya us waqt, apne bhaai ke paas gaya,
bhaai baDaa 'producer' tha us waqt. Ashok Kumar saahab, woh insaaf
pasand aadmi the, kehne lage ki 'bhai yeh 'music director' ki baat
hai, main usmein kuchh naheen bol saktaa, tum Nayyar saahab se baat
karo'.
hamne bolaa ki yeh to Mohd Rafi gaayegaa yaa doosra gaayega

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KS: aapse DaanT khaayi hogi kisi 'singer' ne?

OP: naheen, main DaanT-ta naheen hoon kisi 'singer' ko, sab achchhe
log the, bas ek aadmi ne DaanT khaayi, Kishore Kumar ne.

KS: (bursts into laughter) kaun saa gaana tha woh?

OP: woh "savere ka sooraj tumhaare liye hai", to 'recording' chalti
thi, to do 'regearsal' karke bhaag gaya, to 'recording' ka waqt aa
gaya.

KS: 'naughty' to the hi na woh!

OP: haan, to woh gaaye, to 'musicians' baiThe huye the, to 'mike' se
bolaa andar se 'kis waqt aa rahe ho?'. to bole ki 'mujhe bulaaya hi
kyun?'. maine kahaa ki 'tumhaare maama log ne', kyunki yeh jo
Filmalaya jo tha, Ek Baar Muskurado, yeh unke maama logon ki film thi.

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Song: savere ka sooraj tumhaare liye (Ek Baar Muskurado,1972)
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Singing ability was not the issue at all. OP Nayar was very strict about punctuality and punished ANYONE who was late - even his favorite singers Rafi & Asha. When Rafi came 2 hours late - OP Nayar refused to work with him for 2 years and even then broke the streak because the humble Rafi approached him. Asha would get a stern, public scolding in front of the musicians. Also, OP Nayar was very biased towards Rafi, even at the cost of not utilizing the other great singers to get a most effective variety of the greats. My guess is that Kishore thought this punishment was an overreaction (rather than specifically & directly wanting to sing a particular song) and since his brother was incidentally the producer too, he'd ask him to oversee it (I'd have done the same exact thing). It is possible that what I'm saying Nayar Sahab's earlier comments pertain to another incident with another song and we're mixing them up hence the inconsistency. In any case, it is a TRIVIAL matter (albeit interesting). I do wish Kishore had accepted more songs from MDs in the 50s.