What is the ruling on uttering the phrase "ṣallayt-li" (صليتلي) and similar expressions that are intended to mean "ṣallayt yawsan" (صليت يوماً – you prayed one day), and which are feared to be outwardly shirk?

The common phrase mentioned is not shirk (polytheism), and it is among the questioner's [baseless] anxieties. It is advisable to disregard such thoughts. Utterances whose outward meaning is disbelief, if they occur on the tongue without intending their meaning, do not lead to the one who said them being judged by their implication. An example of this is the saying of the man who, out of extreme joy, mistakenly said: "O Allah, You are my slave, and I am Your Lord." He did not commit disbelief by that. For error, forgetfulness, oversight, slips of the tongue with words one does not intend, and speaking under duress or in ignorance of its meaning—all of these do not lead to accountability, due to the absence of the heart's intention, for which Allah holds a servant accountable.