1Thus have I heard. On one occasion the Blessed One was dwelling at Vesālī in the Great Wood in the Hall with the Peaked Roof. There the Blessed One addressed the bhikkhus thus: "Bhikkhus!"

"Venerable sir!" those bhikkhus replied. The Blessed One said this:

2"Bhikkhus, now the Licchavis dwell using blocks of wood as cushions; sn.ii.268 they are diligent and ardent in exercise. King Ajatasattu of Magadha, the Videhan son, does not gain access to them; he does not get a hold on them. But in the future the Licchavis will become delicate, with soft and tender hands and feet; they will sleep until sunrise on soft beds with pillows of cotton wool. Then King Ajatasattu of Magadha will gain access to them; then he will get a hold on them.

3"Bhikkhus, now the bhikkhus dwell using blocks of wood as cushions; they are diligent and ardent in striving. Māra the Evil One does not gain access to them; he does not get a hold on them. But in the future the bhikkhus will become delicate, with soft and tender hands and feet; they will sleep until sunrise on soft beds with pillows of cotton wool. Then Māra the Evil One will gain access to them; he will get a hold on them.

"Therefore, bhikkhus, you should train yourselves thus: ‘Using blocks of wood as cushions, we will dwell diligent and ardent in striving.’ Thus should you train yourselves." [n.370] "Block of wood" is kaliṅgara. Spk: In the first period of the Buddha’s ministry the bhikkhus would practise meditation from the time they finished their meal (before noon) through the first watch of the night. They would sleep in the middle watch, resting their heads on pieces of wood (kaṭṭhakaṇḍa, a gloss on kaliṅgara); then they would rise early and resume their walking meditation.
The mood of this sutta is similar to the "fears of the future" suttas, AN III 105–10.

1Evaṁ me sutaṁ—​ ekaṁ samayaṁ bhagavā vesāliyaṁ vihārati mahāvane kūṭāgārasālāyaṁ. Tatra kho bhagavā bhikkhū āmantesi: "bhikkhavo"ti.

"Bhadante"ti te bhikkhū bhagavato paccassosuṁ. Bhagavā etadavoca:

2"Kaliṅgarūpadhānā, bhikkhave, etarahi licchavī vihāranti appamattā ātāpino upāsanasmiṁ. Tesaṁ rājā māgadho ajātasattu vedehiputto na labhati otāraṁ na labhati ārammaṇaṁ. Bhavissanti, bhikkhave, anāgatamaddhānaṁ licchavī sukhumālā mudutalunahatthapādā te mudukāsu seyyāsu tūlabibbohanāsu yāvasūriyuggamanā seyyaṁ kappissanti. Tesaṁ rājā māgadho ajātasattu vedehiputto lacchati otāraṁ lacchati ārammaṇaṁ.

3Kaliṅgarūpadhānā, bhikkhave, etarahi bhikkhū vihāranti appamattā ātāpino padhānasmiṁ. Tesaṁ māro pāpimā na labhati otāraṁ na labhati ārammaṇaṁ. Bhavissanti, bhikkhave, anāgatamaddhānaṁ bhikkhū sukhumā mudutalunahatthapādā. Te mudukāsu seyyāsu tūlabibbohanāsu yāvasūriyuggamanā seyyaṁ kappissanti. Tesaṁ māro pāpimā lacchati otāraṁ lacchati ārammaṇaṁ.

Tasmātiha, bhikkhave, evaṁ sikkhitabbaṁ: ‘kaliṅgarūpadhānā viharissāma appamattā ātāpino padhānasmin’ti. Evañhi vo, bhikkhave, sikkhitabban"ti.

Aṭṭhamaṁ.