Seeking sources on -- practicing for this life alone is not even Dharma
The closest I can think of is the Dana sutta in which the Buddha explains how the purity of one's motives have an effect on the results.

&quot;Sariputta, there is the case where a person gives a gift seeking his own profit, with a mind attached [to the reward], seeking to store up for himself [with the thought], 'I'll enjoy this after death.' He gives his gift — food, drink, clothing, a vehicle; a garland, perfume, &amp; ointment; bedding, shelter, &amp; a lamp — to a brahman or a contemplative. What do you think, Sariputta? Might a person give such a gift as this?&quot;
&quot;Yes, lord.&quot;
&quot;Having given this gift seeking his own profit — with a mind attached [to the reward], seeking to store up for himself, [with the thought], 'I'll enjoy this after death' — on the break-up of the body, after death, he reappears in the company of the Four Great Kings. Then, having exhausted that action, that power, that status, that sovereignty, he is a returner, coming back to this world.
[...]
&quot;Or, instead of thinking, 'When this gift of mine is given, it makes the mind serene. Gratification &amp; joy arise,' he gives a gift with the thought, 'This is an ornament for the mind, a support for the mind.' He gives his gift — food, drink, clothing, a vehicle; a garland, perfume, &amp; ointment; bedding, shelter, &amp; a lamp — to a brahman or a contemplative. What do you think, Sariputta? Might a person give such a gift as this?&quot;
&quot;Yes, lord.&quot;
&quot;Having given this, not seeking his own profit, not with a mind attached [to the reward], not seeking to store up for himself, nor [with the thought], 'I'll enjoy this after death,'
[...]
&quot; — but with the thought, 'This is an ornament for the mind, a support for the mind' — on the break-up of the body, after death, he reappears in the company of Brahma's Retinue. Then, having exhausted that action, that power, that status, that sovereignty, he is a non-returner. He does not come back to this world.
&quot;This, Sariputta, is the cause, this is the reason, why a person gives a gift of a certain sort and it does not bear great fruit or great benefit, whereas another person gives a gift of the same sort and it bears great fruit and great benefit.&quot;

-- AN 7.49
Now, if you're asking about the possibility of practicing the Dhamma without, at the very least, assuming rebirth to be true, then in the Apannaka sutta the Buddha indeed said this was wrong view, and thus against the path.

Because there actually is the next world, the view of one who thinks, 'There is no next world' is his wrong view... when he says that 'There is no next world,' he makes himself an opponent to those arahants who know the next world. Because there actually is the next world, when he persuades another that 'There is no next world,' that is persuasion in what is not true Dhamma. And in that persuasion in what is not true Dhamma, he exalts himself and disparages others. Whatever good habituation he previously had is abandoned, while bad habituation is manifested.

-- MN 60